The present application relates generally to apparatus, methods and/or systems concerning the design, manufacture, and use of rotor blades in combustion or gas turbine engines. More specifically, but not by way of limitation, the present application relates to apparatus and assemblies pertaining to turbine rotor blades having midspan shrouds.
In combustion or gas turbine engines (hereinafter “gas turbines”), it is well known that air pressurized in a compressor is used to combust fuel in a combustor to generate a flow of hot combustion gases, whereupon the gases flow downstream through one or more turbines so that energy can be extracted therefrom. In accordance with such engines, generally, rows of circumferentially spaced rotor blades extend radially outwardly from a supporting rotor disc. Each rotor blade typically includes a dovetail that permits assembly and disassembly of the blade in a corresponding dovetail slot in the rotor disc, as well as an airfoil that extends radially outwardly from the dovetail and interacts with the flow of the working fluid through the engine. The airfoil has a concave pressure side and convex suction side extending axially between corresponding leading and trailing edges, and radially between a root and a tip. It will be understood that the blade tip is spaced closely to a radially outer stationary surface for minimizing leakage therebetween of the combustion gases flowing downstream between the turbine blades.
Shrouds at the tip of the airfoil or “tip shrouds” often are implemented on aftward stages or rotor blades to provide a point of contact at the tip, manage bucket vibration frequencies, enable a damping source, and to reduce the over-tip leakage of the working fluid. Given the length of the rotor blades in the aftward stages, the damping function of the tip shrouds provides a significant benefit to durability. However, taking full advantage of the benefits is difficult considering the weight that the tip shroud adds to the assembly and the other design criteria, which include enduring thousands of hours of operation exposed to high temperatures and extreme mechanical loads. Thus, while large tip shrouds are desirable because of the effective manner in which they seal the gas path and the stable connections they form between neighboring rotor blades, it will be appreciated that such shrouds are troublesome because of the increased pull loads on the rotor blade, particularly at the base of the airfoil because it must support the entire load of blade.
One way to address this is to position the shroud lower on the airfoil. That is to say, instead of adding the shroud to the tip of the airfoil, the shroud is positioned near the middle radial region. As used herein, such shrouds will be referred to as a “midspan shrouds.” At this lower (i.e., more inboard) radius, the mass of the shroud causes a reduced level of stress to the rotor blade. However, several issues related to the design and usage of conventional midspan shrouds have been identified by the present inventors. These generally concern the performance of contact wear surfaces or pads that are included between midspan shrouds as a means of mechanically engaging neighboring airfoils for structural and other advantages. Even when robust, these contact surfaces wear quickly given the tendency of misalignment. Such misalignment results in the application of tensional and shear forces to the contact surfaces, which, because the contact surfaces are typically non-integral pads affixed to a surface of the midspan shroud, results in harmful wear that can quickly degrade the component. Additional issues, such as aerodynamic losses, also may result from such misalignment. Finally, to the extent that the weight of such shrouds may be reduced while still fulfilling structural criteria, the life of the rotor blade may be extended.
As will be appreciated, according to these and other criteria, the design of shrouded rotor blades includes many complex, often competing considerations. Novel designs that balance these in a manner that optimizes or enhances one or more desired performance criteria—while still adequately promoting structural robustness, part-life longevity, manufacturability, and/or cost-effective engine operation—represent economically valuable technology.